Just what I wanted. There must be millions of people who don't realise their searches could be more nationally specific and not resticted to uk sites only. Amazing that this is tucked away and not known about more.
As everyone else has said, this does exactly what it says on the tin. I got this after Firefox started defaulting to google.com, which is not great for localised news / law / shopping searches. I've just discovered that I can set this to be the default search engine in the address bar too: go to about:config and change
browser.search.defaultenginename to Google UK - the web
which particularly frustrated me after Firefox broke the Ctrl+E shortcut (though I've just found a plugin to change that too). Happy now. :)
YES! I seem to have spent ages fighting with it. The other UK-ones (because they only return UK results), fail to recognise that Wikipedia even exists. BUT with the default Google search set on Firefox, when you search for "apple imac", you get a load of US prices in the shopping section. This resolves those issues.
A known problem with the search bar for Google is its automatic default to local languages, even when Google's language preferences are set to English. Adding this little tool sorts the problem instantly!